1. Field of the Invention
The present invention relates to a piezoelectric resonating element, a piezoelectric resonating device employing such an element and also to a method for manufacturing such an element.
2. Description of the Prior Art
Generally, the piezoelectric resonating element includes a piezoelectric body and electrodes deposited on the piezoelectric body, while the piezoelectric resonating device includes a casing in which the piezoelectric resonating element is accommodated and terminals connected to the electrodes for the external connection.
The piezoelectric resonating device is, for example, used in a filter of a receiver for receiving AM signal having a central frequency of several 100s KHz, such as 450 KHz. According to the prior art, the arrangement of such a filter can be divided into three types from the view point of mass-production.
The first type is Jaumann type filter which requires an IF (intermediate frequency) transformer and, therefore, has such disadvantages that (i) the impedance matching between the IF transformer and the piezoelectric resonating element is very difficult, (ii) since the transformer includes coils, the inductance varies greatly and, therefore, it is less reliable, and (iii) the employment of the transformer results in bulky in size, and high manufacturing cost. An example of Jaumann type filter is disclosed in the specification of Japanese Utility Model Publication (Jikkosho) No. 47-40256 issued Dec. 6, 1972.
The second type is a 3-terminal type filter which has a shape of square plate and vibrates under expansion mode, or has a shape of circle plate and vibrates under radial expansion mode. The 3-terminal type filter takes such disadvantages that (i) it has a relatively large size, particularly in the case where the third harmonics in the circle plate filter is utilized, the size of the circle plate filter would be considerably large to obtain node points to support the circle plate filter, and (ii) in the case of rectangular plate, unwanted spurious modes of vibrations are produced by the edge mode of vibration. An example of the second type is disclosed in the specification of U.S. Pat. No. 4,360,754 issued Nov. 23, 1982 to Isao Toyoshima et al.
The third type is a filter that has a pair of elongated rectangular plates and vibrates in lengthwise direction under double-mode in which the frequency of vibration between the two plates is slightly different. This type of filter has such disadvantages that (i) it is difficult to adjust the vibrating frequency of each rectangular plate and the coupling between the two rectangular plates, (ii) its structure is complicated, and (iii) its manufacturing cost is high. An example of the third type is disclosed in the specification of Japanese Utility Model Publication (Jikkosho) No. 56-19465 issued May 8, 1981.